The present invention relates to apparatus and methods for inspecting the surface of a substrate such as reticles, photomasks, wafers and the like (hereafter referred to generally as photomasks). More particularly, the present invention relates to an optical inspection system that can scan such a substrate at a high speed and with a high degree of sensitivity.
Integrated circuits are made by photolithographic processes, which use photomasks or reticles and an associated light source to project a circuit image onto a silicon wafer. The presence of defects on the surfaces of the photomasks is highly undesirable and adversely affects the resulting circuits. The defects can be due to, but not limited to, a portion of the pattern being absent from an area where it is intended to be present, a portion of the pattern being present in an area where it is not intended to be, chemical stains or residues from the photomask manufacturing processes which cause an unintended localized modification of the light transmission property of the photomask, particulate contaminates such as dust, resist flakes, skin flakes, erosion of the photolithographic pattern due to electrostatic discharge, artifacts in the photomask substrate such as pits, scratches, and striations, and localized light transmission errors in the substrate or pattern layer. Since it is inevitable that defects will occur, these defects have to be found and repaired prior to use. Blank substrates can also be inspected for defects prior to patterning. $Methods and apparatus for detecting defects have been around for some time. For example, inspection systems and methods utilizing laser light have been introduced and employed to various degrees to scan the surface of substrates such as photomasks, reticles and wafers. These laser inspection systems and methods generally include a laser source for emitting a laser beam, optics for focusing the laser beam to a scanning spot on the surface of the substrate, a stage for providing translational travel, collection optics for collecting either transmitted and/or reflected light, detectors for detecting either the transmitted and/or reflected light, sampling the signals at precise intervals and using this information to construct a virtual image of the substrate being inspected. By way of example, representative laser inspection systems are described in U.S. Pat. No. 5,563,702 to Emery et al., U.S. Pat. No. 5,737,072 to Emery et al., U.S. Pat. No. 5,572,598 to Wihl et al., and U.S. Pat. No. 6,052,478 to Wihl et al., each of which are incorporated herein by reference.
Although such systems work well, there are continuing efforts to improve their design to provide greater sensitivity and faster scanning speeds. That is, as the complexity of integrated circuits has increased, so has the demand on the inspection process. Both the need for resolving smaller defects and for inspecting larger areas have resulted in much greater magnification requirements and in much greater speed requirements, for example, in terms of number of pixels (picture elements) per second processed.
In view of the foregoing, there is a need for improved inspection techniques that provide increased scanning speeds.
Accordingly, the present invention addresses some of the above problems by providing improved apparatus and methods for performing an inspection. In general terms, the inspection system includes components arranged to generate a plurality of beams incident on a sample, such as a photomask. The inspection system also includes components for collecting and detecting a plurality of beams that are reflected or transmitted from the sample as a result of the incident beams.
In one embodiment, an optical inspection system for inspecting the surface of a substrate is disclosed. The inspection system includes a light source for emitting a light beam along an optical axis and a first set of optical elements arranged for separating the light beam into a plurality of light beams, directing the plurality of light beams to intersection with the surface of the substrate, and focusing the plurality of light beams to a plurality of scanning spots on the surface of the substrate. The inspection system also includes a second set of optical elements adapted for collecting a plurality of transmitted light beams caused by the intersection of the plurality of light beams with the surface of the substrate and a light detector arrangement including individual light detectors that each receive individual ones of the plurality of transmitted light beams. The light detectors are arranged for sensing the light intensity of the transmitted light beams.
In a specific implementation, the second set of optical elements includes transmitted light optics for collecting the transmitted light beams that pass through the substrate under inspection. In a more specific implementation, the transmitted light optics include at least a lens arrangement and a prism. The lens arrangement is arranged for focusing the plurality of transmitted light beams onto the prism, and the prism is arranged for directing individual ones of the transmitted light beams towards individual detectors of the light detector arrangement.
In one embodiment, the lens arrangement includes a first transmitted lens and a spherical aberration correction lens. The first transmitted lens are arranged for collecting and focusing the plurality of transmitted light beams, and the spherical aberration correction lens are arranged for maintaining image quality of the plurality of transmitted light beams. In one implementation, the prism includes a plurality of facets that correspond to each of the plurality of transmitted light beams. Each facet is arranged for directing an individual one of the plurality of light beams towards a corresponding individual light detector of the light detector arrangement.
In an alternative embodiment, another optical inspection system for inspecting a surface of a substrate is disclosed. The inspection system includes a transmitted light optical arrangement disposed along an optical axis. The transmitted optical arrangement includes at least a first lens and second lens, both of which are arranged to move along the optical axis, and a prism, which is arranged to move orthogonal to the optical axis. The transmitted light optical arrangement is arranged for directing a plurality of transmitted light beams, which are formed by scanning the substrate with a plurality of light beams, onto a plurality of light detectors. The inspection system further includes a control system for controlling the transmitted light optical arrangement so as to maintain separation of the plurality of beams on the light detectors. The control system includes at least a first lens drive, a second lens drive, and a prism drive. These control components are coupled to a control interface, which is arranged to control the drives. The first lens drive is arranged to actuate the movement of the first lens, and the second lens drive is arranged to actuate the movement of the second lens. The prism drive is arranged to actuate the movement of the prism.